Certain heterostructure materials, such as Aluminum Gallium Arsenide (AlGaAs) and Indium Gallium Arsenide (InGaAs), create an energy well in the conduction band between the two dissimilar heterostructure materials. As a result, free carriers such as electrons or holes, may accumulate in the energy well forming a sheet of charge (i.e., a sheet of electrons). The resulting sheet of electrons that forms in the energy well are typically referred to as a Two-Dimensional Electron Gas (“2DEG”) channel, while a sheet of hole carriers that forms is referred to as a Two-Dimensional Hole GaAs (“2DHG”) channel. FETs that operate by generating and controlling the electrons in the 2D×G channel are conventionally referred to as high electron mobility transistors (“HEMTs”).
A metal-semiconductor field-effect transistor (MESFET) utilizes a Schottky (metal-semiconductor) junction to deplete charge carriers in a semiconductor channel layer. MESFETs typically differ from a typical insulated gate FET or MOSFET in that there is no insulator under the gate over the active switching region, but typically share the feature of a gate contact that extends over the channel layer region. An aspect of this design is the gate metal contacts the 2D×G channel laterally and extends over the channel layer region, forming a Heterodimension metal-semiconductor junction. Generally, narrower the gate the lower the carrier transit time is within the semiconductor channel, which provides higher frequency RF current gain and RF transistor switching performance. MESFET current handling increases with carrier density within the semiconductor channel, but increased carrier density (ie, channel doping) decreases carrier mobility due to increased scattering. Increased channel doping/carrier density also requires that the gate metal contact be elongated in order to provide sufficient charge control. Therefore, in a typical MESFET a trade exists between current handling (current density), carrier transit time (ie high frequency) performance. As a result, most production MESFETs are either optimized for higher current density applications, such as low frequency RF power amplifier, RF switch, and high linearity mixers, or optimized for high frequency performance such as low-noise amplifier, or wideband RF switches. High frequency MESFET also uses a built up top layer of low resistance metal on the gate, often producing a mushroom-like profile in cross section. Building upon the basic structure of the HEMT, a metal-semiconductor heterodimension field effect transistor (MESHFET) has been developed that takes advantage of the heterostructure materials to increase electron mobility within the MESFET while also overcoming the trade off between carrier density and carrier transit time that exists for conventional MESFET. The lateral gate to 2D×G contact provides sufficient charge control even as the carrier density is increased with a gate length narrowed. MESHFETs have been used for microwave frequency communications and radar applications. The combination of MESHFET and HEMT in a monolithic integrated circuit provides optimal RF performance for multi-function designs that combine low-noise amplifiers, power amplifiers, low-loss and wideband RF switches, and high linearity mixers and amplifiers.